1. Technical Field
The present disclosure relates to a technology for outputting a gamma voltage in a source driver of a display device, and more particular, to a gamma voltage generation circuit of a source driver, which is capable of forming a high gamma voltage range and a low gamma voltage range even when a negative power supply voltage and a positive power supply voltage are asymmetrical to each other.
2. Related Art
In general, a display device includes a source driver which drives data lines of a display panel according to R, G and B data inputted from outside.
The source driver of the display device is configured to generate gamma voltages using a positive power supply voltage and a negative power supply voltage which are provided from a power supply, and select gamma voltages corresponding to R, G, and B data in order to output a data signal.
For example, the power supply may generate a positive power supply voltage VSP, and generate a negative power supply voltage VSN using the positive power supply voltage VSP. On the other hand, the power supply may generate a negative power supply voltage VSN, and generate a positive power supply voltage VSP using the negative power supply voltage VSN.
At this time, the positive power supply voltage VSP and the negative power supply voltage VSN, which are supplied to the source driver, may be set in such a manner that the negative power supply voltage VSN has a larger absolute value or the positive supply voltage VSP has a larger absolute value, from problem like the efficiency of the circuit. As such, the positive power supply voltage VSP and the negative power supply voltage VSN, which are asymmetrical to each other based on a ground voltage GND, may be supplied to the source driver.
The positive power supply voltage VSP and the negative power supply voltage VSN may be used to generate a plurality of reference voltages, and the plurality of reference voltages may be divided into the equal numbers of high reference voltages and low reference voltages, based on an intermediate value between a positive level voltage and a negative level voltage.
Among the plurality of reference voltages, a reference voltage having the closest level to the positive power supply voltage may be referred to as the most significant high reference voltage, and a reference voltage having the closest level to the negative power supply voltage may be referred to as the most significant low reference voltage. Reference voltages having the closest voltage level to the intermediate value between the positive power supply voltage and the negative power supply voltage may be referred to as the least significant high reference voltage and the least significant low reference voltage, respectively.
The average voltage of the least significant high reference voltage and the least significant low reference voltage may be referred to as a virtual ground voltage VG.
In response to the high reference voltages, high reference gamma voltages are generated. The high reference gamma voltages may be divided to generate high gamma voltages. In response to the low reference voltages, low reference gamma voltages are generated. The low reference gamma voltages may be divided to generate low gamma voltages. The high gamma voltages may be formed in a high gamma voltage range which is defined as a higher level than the virtual ground voltage VG, and the low gamma voltages may be formed in a low gamma voltage range which is defined as a lower level than the virtual ground voltage VG. When the absolute value of the positive power supply voltage VSP supplied to the source driver is larger than the absolute value of the negative power supply voltage VSN, the virtual ground voltage VG is formed to be higher than the ground voltage GND. That is, one or more of the reference voltages included in the positive region, for example, the least significant low reference voltage may be positioned in the positive region. The least significant low reference gamma voltage and the least significant low gamma voltage, which correspond to the least significant low reference voltage, also exist in the positive region.
Gamma buffers which receive high reference voltages and stably output high reference gamma voltages are driven using the positive power supply voltage and the ground voltage, and gamma buffers which receive low reference voltages and stably output low reference gamma voltages are driven using the ground voltage and the negative power supply voltage.
However, when the least significant low reference voltage is positioned in the positive region such that a low reference voltage having a positive value is received, the corresponding gamma buffer is driven using the ground voltage GND and the negative power supply voltage VSN. Thus, the gamma buffer may have difficulties in outputting a normal low reference gamma voltage. When the high gamma voltage range and the low gamma voltage range are set to be symmetrical with each other in order to solve the above-described problem, an available gamma voltage range may be reduced.
Furthermore, when the absolute value of the negative power supply voltage VSN supplied to the source driver is higher than the absolute value of the positive power supply voltage VSP, a similar problem may also occur.